2 Networks Starting in 1876, the public switched telephone network (PSTN) has become a global infrastructure for voice communications Long-haul network PBX Toll switch (Backbone switch) and Local Loop Long-haul network PBX Toll switch Each phone user (subscriber) has a direct connection to a switch in the central office. This is called the local loop The local loop has a length of 1-10 km The switches in the central office are called (local) exchange A company which provides local telephone service is called a local exchange carrier or LEC (e.g., Bell Atlantic)

3 PBX Long-haul network PBX Toll switch A PBX (Private Branch Exchange) is a telephone system within an enterprise that switches calls within the enterprise on local lines, while allowing all users to share a certain number of external lines to the central office The main purpose of a PBX is to save the cost of requiring a line for each user to the telephone company's central office. The long-haul network Long-haul network PBX Toll switch Toll or backbone switches provide long-distance connectivity over longdistance trunks There are only about 500 toll switches in the United States. Each toll switch can run more than 100,000 simultaneous phone calls

4 How is voice transmitted? Voice can be transmitted in two ways: Analog voice transmission: Each voice channel is allocated a bandwidth of 3.5 khz Digital voice transmission: Analog voice stream is converted in a digital stream: Standard scheme for a voice call: Obtain 8000 samples per second, each with length 8 bit How is voice transmitted? Until 1960s: Entire telephone network is analog Frequency-division multiplexing Today: The local loop is analog. The rest of the network is digital (based on TDM) All digital: When do we get an all digital network? ISDN (Integrated services Digital Network) is an all digital circuitswitching technology. ISDN is available since the early-1990s (in Europe) or mid-1990s (US). No wide deployment in US Another all digital but not circuit-switched telephony solution is IP telephony.

7 Addressing and Routing Each subscriber has an address (telephone number) Addresses are hierarchical Example: Domino s Pizza in downtown Charlottesville country code area code number of local exchange subscriber number The information contained in a telephone address is exploited when establishing a route from caller to callee Signaling Pick up Dial Set up Quiescent Offhook Dialing Wait phone Disconnect Disconnecting Put phone down Talking Remote site picks up Setup complete Ringing Signaling refers to the control functions performed to setup a phone call Signaling between users and the local exchange in the central office is quite simple: dial-tone, punch numbers, put phone down, etc. Signaling between exchanges/switches is more complicated and is done via a separate network, which uses packet switching

8 ing in the Network switch has two parts: controller responsible for signaling (control plane) ing hardware responsible for handling voice data (data plane) Signaling Network Signaling between switch controllers is done over a (packet switching) network that is separate from the data plane This is called out-of-band signaling. Packet network is called Signaling System 7 (SS7). SS7 has a complete protocol stack.

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